Self-piercing and riveting fasteners of the type disclosed in U.S. Pat. No. 4,555,838, assigned to the assignee of the present application, are now used in mass production applications, including automotive applications. In such mass production applications, the fasteners are normally installed in a die press capable of generating several tons of die force. The die press may be utilized to simultaneously form the panel into a configured shape, such as an automotive body component or structural support member, and install several riveting fasteners.
One or a plurality of installation heads are installed in one of the die platens of the die press, and the opposite die platen includes aligned riveting die members, commonly referred to as die buttons. The riveting fasteners are fed into the installation heads, generally through plastic tubes or chutes. The installation heads include a feed mechanism, which feeds a fastener to a plunger or punch, reciprocal in the installation head, to install a fastener in a panel located in the die press, with each stroke of the die press.
The self-piercing and riveting fasteners of the type described herein generally include a body portion and an integral tubular or annular barrel portion. In a stud-type fastener of the type disclosed, for example, in the above-referenced U.S. Pat. No. 4,555,838, the body portion of the fastener includes a radial flange integrally joined to the annular barrel portion opposite the free end and a stud portion integrally joined to the flange portion extending in coaxial alignment with the barrel portion. The free end of the tubular barrel portion is open and includes a piercing surface. The opposite end of the barrel portion is closed at the radial flange portion, forming a socket. The die button includes an annular concave die cavity surrounding a central die post. As disclosed, for example, in the above-referenced U.S. Pat. No. 4,555,838, the central die post includes a free end having a peripheral piercing surface and an outer surface which slopes radially outwardly into the annular concave die cavity.
A panel, such as a sheet steel panel or plate used for body panels and structural members by the automotive industry, is secured over the die member, preferably in a die press, as described above. The free end of the barrel portion is then driven into the panel in coaxial alignment with the central die post of the die button. The die post is configured to be received within the free end of the annular barrel portion. As the barrel portion is driven into the panel, the piercing surface at the free end of the panel pierces a slug from the panel, which is received on the free end of the die post of the die button. The barrel portion is then driven through the opening formed in the panel into the annular die cavity, radially deforming the free end of the barrel portion, riveting the barrel portion to the panel as described, for example, in the above-referenced U.S. Pat. No. 4,555,838.
When the self-piercing and riveting fastener of the type disclosed in the above-referenced patent was first introduced, several problems were experienced in producing a consistent, high-quality installation. A primary object of this fastening system is to provide a stud or nut-type fastener permanently installed in relatively thin metal panels, such as utilized by the automotive industry, which may have a thickness of 0.030 inches or less. In the most preferred embodiment of the self-piercing and riveting fastener, the tubular barrel portion is deformed radially outwardly in the annular concave die cavity into a U-shaped channel opening toward the body portion of the fastener, and the panel portion surrounding the pierced opening is simultaneously driven into the U-shaped channel and deformed to substantially fill the channel, forming a very secure mechanical interlock between the fastener and the panel. As the slug is pierced from the panel, the slug is received in the tubular barrel portion and finally driven to the bottom of the tubular barrel portion by the central post of the die button.
It was initially believed that the location of the slug in the tubular barrel portion was essential to prevent inward collapse of the barrel portion, as the barrel portion is deformed radially outwardly into the preferred U-shaped channel because of inwardly directed forces on the barrel portion as described, for example, in the '838 patent. Although the slug does support the barrel portion during radial deformation, it has been found that it is possible to form a secure installation without locating the slug in the barrel portion based upon improvements in the installation, as described below. Nevertheless, the preferred embodiment of the stud-type fastener and panel assembly includes the slug in the barrel portion, particularly in mass production installations, such as automotive applications. It is far easier to retain the slug in the assembly than find a way of removing the slug through the die button, for example. Further, it may not be possible to remove the slug in certain existing applications because of lack of access. In a typical automotive application, studs are attached to automotive structural components or panels in a die press, as described. Further, several studs may be installed simultaneously, and the die press preferably is run continuously during a production run. Therefore, a substantial volume of panel slugs must be removed from the die press area unless the slugs are retained in the assembly.
The initial problem with forming the preferred embodiment of the self-piercing and riveting fastener and panel assembly resulted from tolerance control and friction between the concave annular die cavity surface and the tubular barrel portion as the tubular barrel portion is initially deformed radially outwardly, as described in U.S. Pat. No. 4,825,525, assigned to the assignee of the present application. As described in this patent, the required installation force in a typical application is between about 10 and 20 tons. Particularly in mass production applications, orientation of the self-attaching element and die member and very close tolerances are critical to achieving a good installation. The installation force, which is composed of the forces required for piercing, forming of the barrel, and setting of the panel, results in very significant friction between the barrel portion of the fastener and the die cavity. The pressure between the internal surface of the tubular barrel portion and the die cavity may exceed 50 tons per square inch. The frictional forces generated by this pressure sometimes resulted in collapse of the barrel portion or stud, poor or incomplete installations and stress risers or faults, which significantly reduced the integrity of the joint assembly. These problems were, however, overcome by coating at least the internal surface of the barrel portion with a friction-resistant coating, preferably a fluorocarbon coating, such as an air-drying polytetrafluoroethylene lubricant, suspended in a fast-drying thermoplastic resin as disclosed in U.S. Pat. No. 4,825,525.
Substantial improvements in manufacturing tolerances and the use of a friction-resistant coating has resulted in more consistent installations, including an excellent mechanical interlock between the fastener barrel portion and the panel. In fact, the pull-out strength of a stud fastener as disclosed in the above-referenced U.S. patents may be greater than the tensile strength of the stud, in many applications. These improvements have, however, resulted in another problem, namely, the retention of the panel slug in the barrel portion. The panel slug may drop out of the barrel portion as the die press is opened or during assembly of the structural element to the panel on which the stud fastener is attached. Where the slug is released during installation of the stud-type fastener, the slug may remain on the center post of the female die member. When a second stud is then installed in a new panel received in the press, a slug is pierced from the panel and received over the die button center post, over the first slug. This results in damage to the installation head or deformation of the flange portion of the second stud, which may actually be sheared from the fastener by the installation plunger. Thus, it is very important to securely retain the panel slug in the barrel portion of the stud-type fastener during installation. The problem of retaining the slug in the barrel portion is particularly difficult where the panel is relatively thin, such as automotive applications where the panel metal may be less than 0.040 inches or less.
The problem of slug retention has been solved by the method of attaching a self-piercing and riveting fastener of this invention and the improved die button by securely staking the panel slug in the barrel portion. As will be understood, the slug must be retained in the barrel portion during installation, avoiding the necessity of a separate step. The method of this invention and the improved die button stakes the slug in the barrel portion during installation of the fastener in the panel, thus eliminating a separate step.